Support private gitlab repo as git submodule

[lmilbaum]

How are you running Renovate?

Self-hosted Renovate

If you're self-hosting Renovate, tell us which platform (GitHub, GitLab, etc) and which version of Renovate.

GitLab

Please tell us more about your question or problem

I'm sure I mentioned it in the past in one of the discussions.
According to my experience, this use case is not supported. I'm unable to generate a reproduction repo as the the referenced gitlab repos are private.
I have reason to believe that gitlab supports the insteadOf feature. Could you please advice how to contribute a fix?

Logs (if relevant)

Logs

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[rarkins]

It's been a long time since I looked into this code.

Are you using cloneSubmodules, or just wanting Renovate to be able to update the submodule digest using the git-submodules manager?